A typical high power, radio frequency (RF) semiconductor device such as a RF power transistor may include one or more input leads, one or more output leads, one or more transistors, bondwires coupling the input lead(s) to the transistor(s), and bondwires coupling the transistor(s) to the output lead(s). The bondwires have significant reactances at high frequencies, and the associated inductances may be factored into the design of input and output impedance matching circuits for a device. In some cases, input and output impedance matching circuits may be contained within the same package that contains the device's transistor(s). More specifically, an in-package, input impedance matching circuit may be coupled between a device's input lead and a control terminal (e.g., the gate) of a transistor, and an in-package, output impedance matching circuit may be coupled between a current conducting terminal (e.g., the drain) of a transistor and a device's output lead. Each of the input and output impedance matching circuits may include one or more capacitive and resistive elements, along with the inductances inherent in the sets of bondwires interconnecting those elements with the device's transistor(s) and with the input and output leads.
Such packaged RF semiconductor devices are readily available, which have very good performance when used in narrow-band applications. However, designing suitable packaged RF semiconductor devices for wideband, multi-band, and/or multi-mode operation is challenging for several reasons. For example, in a packaged RF semiconductor device, the lead level output impedance is limited by the number of matching sections. Therefore, to achieve an acceptable lead level output impedance for a wideband, multi-band, and/or multi-mode application, it may be desirable to incorporate multiple, in-package matching sections. However, the inclusion of multiple matching sections in a device increases the number of impedance matching elements in the impedance matching circuits, and thus increases the size of the device. In addition, the various sets of bondwires that would be implemented to interconnect the impedance matching elements for multi-stage matching may create unacceptable inductive coupling between the matching sections, which may limit the effectiveness of the impedance transformation. In addition, to facilitate good performance for wideband, multi-band, and/or multi-mode implementations, relatively large discrete capacitors in the impedance matching circuits may be warranted. Accordingly, in order to accommodate the relatively large capacitors, package sizes for such implementations would need to be further increased. Increasing semiconductor device package size is incompatible with the industry trend to reduce device sizes and costs.
Indeed, one of the primary goals of any new generation of radio frequency (RF) power transistors is to achieve higher output power levels in smaller package footprints. Towards achieving this goal, the terminal impedances of the transistor tend to get lower with an increase in the transistor periphery. Relatedly, with lower terminal impedances, it becomes desirable to use multiple matching sections to raise these impedances to an acceptable level. In particular, the goal of a pre-matching network is to transform a low impedance to a level that facilitates matching on a printed circuit board (PCB) up to the system reference impedance. Additionally, the need for wide-band instantaneous operation necessitates lowering the base-band impedance seen by the power transistor. A wideband low impedance envelope termination is necessary to maintain low levels of distortion and to facilitate wideband pre-distortion linearization.
Notwithstanding such concerns, current techniques for impedance matching in the context of RF power transistors provide limited improvement with increased assembly complexity. Typically, RF power transistors use resonating inductors as the primary impedance matching element. For higher power levels a second matching section has been used for additional transformation. However, such additional matching sections add loss, manufacturing complexity and impedance dispersion across frequency, which makes such componentry difficult to use in applications such as Doherty power amplifiers. And traditionally the RF package has been an interface from the transistor to the PCB with little or no impedance transforming properties (depending on frequency). Likewise, current techniques fail to provide desired wideband low impedance envelope termination as is desirable for at least the above-discussed reasons.